Magnetic resonance imaging (MRI) is a medical imaging modality that can create images of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field (i.e., the “main magnetic field”). When a human body, or part of a human body, is placed in the main magnetic field, the nuclear spins that are associated with the hydrogen nuclei in tissue water become polarized. This means that the magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis (the “z axis,” by convention). A MRI system also comprises components called gradient coils that produce smaller amplitude, spatially varying magnetic fields when current is applied to them. Typically, gradient coils are designed to produce a magnetic field component that is aligned along the z axis and that varies linearly in amplitude with position along one of the x, y or z axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength, and concomitantly on the resonance frequency of the nuclear spins, along a single axis. Three gradient coils with orthogonal axes are used to “spatially encode” the MR signal by creating a signature resonance frequency at each location in the body. Radio frequency (RF) coils are used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. These coils are used to add energy to the nuclear spin system in a controlled fashion. As the nuclear spins then relax back to their rest energy state, they give up energy in the form of an RF signal. This signal is detected by the MRI system and is transformed into an image using a computer and known reconstruction algorithms.
MRI data may be acquired using a three-dimensional (3D) acquisition strategy, the most common of which is a rectilinear sampling that fills a 3D Cartesian grid with Fourier reciprocal space (i.e., “k-space”) data. The data may be collected with Nyquist frequency sampling to provide unique location encoding of the MRI signals and thereby prevent aliasing in the reconstructed images. The 3D data is spatially encoded using phase encoding along two perpendicular spatial directions (the y and z directions) and frequency encoding along the third (the x direction). Usually, the secondary phase encoding is referred to as “slice encoding,” to distinguish it from the primary phase-encoding. The resultant raw data fills a 3D k-space matrix which is then “reconstructed” using Fourier transformation techniques, resulting in a stack of two-dimensional images.
MRI data is typically collected in frames that are referred to as “views.” For 3D imaging, each view corresponds to a single ky and kz value, but contains data for the full range of kx values that are required to reconstruct an image. Multiple view-ordering schemes are known in the art for determining how ky, kz encoding is performed for each view. For example, in a “nested” view-ordering scheme, all of the views corresponding to one phase-encoding axis (kz, for example) are acquired before incrementing the value on the other phase-encoding axis (ky, for example). An “elliptical centric” view-ordering scheme replaces the two nested loops with a single loop that steps through ky, kz pairs according to their distance from the origin in the ky-kz plane. The choice of a view-ordering scheme often depends on how the imaged object or its corresponding magnetization is expected to change during the data acquisition. Views near the center of k-space have the strongest effect on the overall image appearance, because most of the k-space information about an object is contained near the center of k-space.
Dynamic or time-resolved MR studies (or acquisitions) have been developed to image dynamic or time-varying processes (e.g., cardiac motion, multi-phase scans, contrast enhancement (e.g., vascular contrast), joint motion, catheter tracking, etc.) and typically involve repeatedly collecting the same data over time. View ordering techniques have been developed for encoding views in a dynamic or time-resolved acquisition, for example, keyhole techniques such as TRICKS (Time Resolved Imaging of Contrast Kinetics) and BRISK (Block Regional Interpolation Scheme for K-Space) that divide k-space into multiple regions and repeatedly update data in the center of k-space more frequently than other parts of k-space. Such techniques, however, can result in eddy current induced artifacts due to intra- and inter-region jumps in k-space during the acquisition. It would be desirable to provide a method and apparatus for acquiring MRI data for a dynamic or time-resolved acquisition that minimizes intra- and inter-region jumps and reduces eddy current induced artifacts.